1. Field of Invention
The invention relates to a double-sided high-density information storage medium and, in particular, to a double-sided high-density information storage medium that uses a short-wavelength laser pickup head to perform data reading and writing.
2. Related Art
There is an increasing demand for high storage density and capacity in the storage media of 3C (computer, communication, consumer electronics) products. Taking the optical information storage medium as an example, the compact disk (CD) specifications proposed by Philips and Sony together in 1982 have been widely accepted and used.
As the multimedia are having wider applications in the coming digital era, higher demands in audio/video (AV) effects are expected from users. Consequently, there is an increasing need for high storage density and capacity in the data storage media. In particular, the traditional CD specifications are no longer sufficient for digital AV effects. Taking the popular video compact disk (VCD) as an example, the screen quality is far worse than the normal video tapes and a single VCD can only record one hour of AV information. Therefore, Philips and Sony reached an agreement and proposed with other companies the specifications of digital versatile discs (DVD's) in April, 1996. The digital data storage capacity of a single DVD is 4.7 GB (Gigabyte), far more than the 650 MB (Megabyte) capacity of a single CD. Therefore, hardware for reading and playing DVD's has emerged and become part of the standard equipment. Many related DVD specifications are also developed in the meantime.
To integrate the current image signal playing modes and to pursue higher AV quality, the high-definition television (HDTV) playing technology has become the most advanced playing mode. Therefore, a whole new AV standard with high AV quality HDTV signals have become invented. HDTV is a new video signal display specification that supports the 1,920×1,080 resolution, uses a 16:9 display screen and a refreshing rate of 30 or 60 screens per second (the refreshing rate of the traditional television systems is 24 or 30). The storage capacity of optical disks thus faces a bottleneck. If 4.7 GB DVD's are used to high-quality AV signals for the HDTV, only 40 minutes of information can be recorded for each disk. The use of DVD's is thus greatly limited in the HDTV epoch.
Red-light laser is usually used as a pickup light source for optical information storage media but there is an optical interference limit that puts restriction on the recording density. The focal spot size of the optical pickup head and the wavelength are correlated. The focal spot size decreases as the wavelength of the pickup laser source becomes shorter, then the recording capacity per unit area is increased several times. The specifications for high-capacity optical storage media, such as the high-density digital versatile disc (HD-DVD), currently proposed by various manufacturing companies ask for blue or violet light with a shorter wavelength for the optical pickup head. From the above description, we see that defining optical disc specifications and structures that use short-wavelength lasers is the main stream of developing optical storage media in future.